1. The Field of the Invention
This invention concerns improvements in and relating to fuel transportation, particularly but not exclusively relating to enriched nuclear fuels.
2. The Relevant Technology
Nuclear fuels, such as enriched uranium or mixed oxide forms, frequently need to be transported between sites, for instance the enrichment site and the fuel rod production site. The fuel is normally in the form of pellets or powder at this stage. International standards apply, requiring certain levels of thermal insulation and structural strength. A major concern is criticality control. The mass of enriched fuel within the transport container must be strictly limited to ensure that a criticality event does not occur. This single requirement places a stringent limit on the volume of fuel which can be transported in any given volume of a transport container. In this regard, the transportation of nuclear fuel differs significantly from transportation of other radioactive materials. Radioactive waste is of a far lower enrichment, thus facilitating transport of greater volumes in proximity with one another. In assembled fuel rods on the other hand, the volume of fuel when compared with the overall fuel rod and supporting structure volume is very low.
Present systems usually consist of a cylindrical drum provided with one or more layers of wood on all sides, the wood defining a central recess into which a single cylinder containing the enriched fuel is placed.
The fuel containing volume of the inner drum is very low compared with the volume of the outer drum. As a consequence the transportation of fuel takes up a considerable amount of space. The commercial considerations of this apply as they do to any transportation procedure. Additionally the cylindrical nature of the unit presents handling and stability problems.
According to a first aspect of the invention we provide a transportable container for nuclear fuel, the container comprising an outer container provided with internal insulation, the insulation defining an internal cavity, the cavity receiving a plurality of fuel containers, wherein the internal volume of the fuel containers is at least 5% of the external volume of the outer container.
A container having this level of fuel volume to overall volume has not previously been achieved. The present invention also allows this level to be reached whilst meeting the necessary criticality, insulation and other standards.
Preferably the internal volume of the fuel container is at least 10% of that of the external volume of the outer container. A level of at least 15%, 20% or 25% is preferred. Levels of at least 30%, 35% or even 40% may be reached. Any increase in fuel volume to overall container volume is significant in reducing transportation costs and the capital costs involved in providing the strong fuel containers.
Preferably the outer container is formed with a steel and most preferably stainless steel skin. The corners and/or edges of the outer container may be provided with strengthening elements. These may take the form of L-shaped sections. The outer container is preferably provided with feet.
The outer container is preferably provided with a lid. The lid is preferably releasably fastened to the outer container. Clamps attached to the outer container and releasably engageable with the periphery of the lid are preferred. The clamps may also be releasably engaged with the outer container. The lid may be provided with handles or other forms of engagement for removal of the lid.
It is particularly preferred that the lid be received within the perimeter of two or more projections from the outer container. The strengthening elements projecting above the top of the container may define this perimeter.
Preferably the insulating material is provided in a series of discrete layers. One or more base layers and/or one or more wall layers for each wall may be provided. The lid insulation may be mounted on the metal lid or may be provided separately. If provided separately a pair of interlinking sections may be provided.
The insulating layer is preferably thermally insulating and/or neutron absorbing. Calcium silicate offers a preferred insulating material. One or more different materials may be used together or in a sandwich style structure.
Preferably the insulation layer defines the boundaries of a single internal cavity. A rectilinear cavity is preferred.
The internal cavity is preferably provided with a correspondingly shaped single unit internal container comprising four side walls and a base. The internal container is preferably made of steel, boronated steel, or most particularly stainless steel.
In one form the internal container is preferably divided up into a series of chambers. The chambers may be defined by one or more elements crossing the internal cavity or container. Preferably the elements are plates spanning the full height, or at least substantially the full height, of the internal volume. Preferably one or more elements span the internal volume in different directions, most preferably at substantially 90 degrees to one another. Preferably the plates are substantially vertically provided. It is particularly preferred that two plates cross the internal cavity in each of two directions at 90 degrees to one another. Preferably the internal volume is divided up into nine substantially equivalent chambers.
In a second alternative form, the internal cavity may be fitted with elements such as plates spanning the full height of the internal volume to define an internal container. The chambers again being defined by one or more elements crossing the internal cavity. A base plate may be provided on the base insulating layer to define a base for the internal volume. A top plate may also be provided. Side plates may also be provided to define the sides of the chambers.
One or more of the base, top or dividing elements or plates may be formed of metal. Steel and in particular stainless steel or boronated steel.
The base, side and dividing plates or elements of the single unit internal container is alternatively provided as a separate unit to the insulating layers and outer container.
In a further alternative form the internal cavity may be fitted with a plurality of sleeve elements. Preferably the sleeve elements are adapted to receive fuel containers or pails. The sleeves may be continuous or substantially continuous. Preferably the sleeves are of circular cross-section. Preferably the sleeves internal diameter is substantially equivalent to the external diameter of the fuel containers or pails. Preferably the sleeves are rigidly separated from one another. The sleeves may be rigidly separated by mounting on a base plate.
Preferably the sleeves are discrete from one another around their entire periphery. Four or more, and preferably 8 or 9 such sleeves may be provided within the internal cavity.
The base plate may be attached to one or more side plates or elements. The side plates or elements may form walls corresponding to the walls of the internal cavity. An internal container may thus be provided.
Preferably one or more of the sleeves are at least in part surrounded by a neutron absorbing material. Preferably one or more of the sleeves, and most preferably all of the sleeves, are surrounded by a neutron absorbing material around their entire circumference. A neutron absorbing material may optionally be provided around one or both ends of one or more of the sleeves.
Preferably the neutron absorbing material is a resin based material. Preferably the neutron absorbing material is fire resistant. Preferably the resin based material is loaded with, up to 6.5% boron, or up to 5% boron, and more preferably up to 2.5% boron. Preferably the resin occupies at least 50% of the non-sleeve volume of the internal cavity. The neutron absorbing material may fill the entire non-sleeve volume of the internal cavity or lower density materials may be incorporated, such as polystyrene.
The internal container is preferably provided with a lid.
Preferably the fuel containers or pails comprise cylindrical drums. Preferably releasable lids are provided. The release mechanism for the lid is preferably contained within the plan profile of the container in the sealed position to minimise space.
The fuel preferably occupies at least 50% of the fuel container and may occupy 60, 70, 80, 90, 95% or any individual % value over 50%.
The fuel may be provided within the fuel containers in plastic bags, such as polyethylene.
The fuel may be in pellet, powder or other form. Unirradiated enriched uranium may be the fuel. The provision of uranium at substantially up to 5% enrichment may be used. A density of around 1.4 g/cm3 may be used. In such a case each individual fuel container may have a volume of between 15 and 20 liters, for instance 17.3 liters.
The boron content of any one of the insulator layers, internal divisions, sleeves, fuel containers, or remaining chamber space may be increased to give increased absorption.
Fuel containers are preferably provided in more than three of the chambers or sleeves. The provision of the fuel containers in peripheral chambers or sleeves and most preferably all the peripheral chambers or sleeves is envisaged. One or more of the chambers or sleeves may be provided with a neutron absorber. Preferably the neutron absorber is provided in a unit corresponding in dimensions to the chamber or sleeve receiving it. The provision of polyethylene as the neutron absorber is preferred. The polyethylene absorber may be in a steel container corresponding to the size and shape of the chamber or sleeve receiving it. The absorber may also be provided with a lid corresponding with the chamber or sleeve into which the absorber is placed in order to assist in retaining the absorber within the chamber or sleeve. The lid is preferably of steel.
In a particularly preferred form the container comprises an outer container with a removable lid, the outer container being provided with an insulating layer on each wall and base, a further removable insulating layer being provided between the lid and the internal cavity of the container in use, the internal cavity being divided into a plurality of chambers, a fuel container being provided in at least three of the chambers and at least one of the chambers being provided with a neutron absorbing material.
In an alternative particularly preferred form the container comprises an outer container with removable lid, the outer container being provided with an insulating layer on each wall and the base, a further removable insulating layer being provided on the lid, the insulating layers defining an internal cavity of the container, the internal cavity being provided with a plurality of sleeves, a fuel container being provided in at least 3 of the sleeves and the sleeves being at least partially surrounded by a neutron absorbing material.
It is preferred that only one fuel container or pail be provided in each chamber.
A particularly preferred arrangement provides a rectangular plan aperture divided into nine chambers, three chambers by three chambers. Preferably the fuel containers are provided in the peripheral chambers. A neutron absorbing material may be substituted in the central chamber and/or one or more of the other chambers as required.
In a further particularly preferred arrangement a rectilinear plan internal cavity may be provided with nine sleeve elements, in a three by three sleeve element arrangement. Preferably fuel containers are provided in all the periphery sleeves and most preferably in all of the sleeves. A neutron absorbing material may be substituted in one or more of the chambers.